1. Field of the Invention
The present invention relates to a driving device, and more particularly, to a lens driving device for transporting a lens of an optical instrument via separate actuation of the lens itself.
2. Description of the Related Art
Various optical instruments such as a camera, a camcorder, a zoom camera, an observation camera, an optics in a Micro Air Vehicle (MAV), etc. have a lens structure which enables reciprocating movement of a lens for zooming. There have been developed several structures for transporting the lens for such zooming.
A conventional driving technique based upon a cam structure has been conventionally used for zooming to vary a focal length. The cam structure-based driving technique executes zooming by varying the relative spacing of lenses along a lens barrel, which is driven by an electromagnetic motor, and a cam-shaped groove placed in a lateral portion of the lens barrel. The cam structure-based zooming technique determines the relative position of each lens according to the configuration of a cam. Therefore, there are drawbacks in that a driving unit and a focusing lens for setting a focus at a specific magnification are further required and a driving mechanism including a driven reduction gear and a lens holding structure which moves along the cam is complicated.
Recently, there is developed a micro optical zoom mechanism in order to overcome the above drawbacks as well as to impart a zooming function to a micro optical instrument. According to the current trend, the micro optical instrument employs an intelligent device such as a piezoelectric element rather than the conventional driving technique using the electromagnetic motor. Substituting the piezoelectric element for the conventional motor driving technique has advantages that a driving structure can be simplified and high efficiency can be realized owing to a direct drive mechanism.
FIG. 1 shows an example of a zoom lens device using such a piezoelectric element which is disclosed in U.S. Pat. No. 6,215,605, entitled “Driving Device.” The lens driving device in FIG. 1 fixes piezoelectric actuators 111 and 112 and transfers the displacement to driving rods 116 and 117 so as to transport lenses L2 and L4 under effects of the preload from projections 131a and 132a and inertia and acceleration of lens holders 131 and 132. The piezoelectric actuator 112, according to the waveform of exciting input, transports the lens by enabling the lens holder to move together with the driving rod or to slide and stay in position. The piezoelectric actuator 112 can also transport the lens through a reciprocating movement.
The lens driving device shown in FIG. 1 is arranged in use as shown in FIG. 2, in which the piezoelectric actuators 211a and 211b are arranged adjacent to each other. So, when a base block 213 receives any expansion/compression transferred from one of the piezoelectric actuators 211a and 211b, this expansion/compression may be also transferred to the other one of the piezoelectric actuators and thus its corresponding lens. Therefore, the base block 213 is provided with a groove 213g in order to block transfer of the expansion/compression between the piezoelectric actuators. However, the groove complicates the structure of the driving device while causing fabrication of the driving device difficult. Furthermore, the groove cannot completely block the expansion/compression interference between the piezoelectric actuators.
Further, the length of the driving rods 116 and 116, which reciprocate by the piezoelectric actuators to transport the lenses, is largely restricted according to the size of the piezoelectric actuators. Such restriction to the length of the driving rods acts as limitation to the transportable distance of the lens and thus disadvantageously affects qualities of a product incorporating the lens driving device.
Because the driving rods are essentially fixed, the conventional lens driving device has following problems: It is impossible to vary the length of the lens barrel on which the lenses are internally mounted. In addition to a space used for transport of the lens, an additional space for arranging driving elements is required. As a result, the overall size of the driving device is rarely reduced. Furthermore, the lens is partially supported by the driving rod so that asymmetric expansion/contraction may occur in the lens during actuation, thereby potentially making the lens driving operation unstable.